System and method for automatic atrial capture detection and atrial pacing threshold determination

ABSTRACT

An implantable dual chamber stimulation device provides a novel detection scheme for automatically detecting atrial capture and performing an atrial pacing threshold assessment. The stimulation device preferably waits until the patient is at or near rest and monitors the patient&#39;s P-wave activity to determine a detection window where a next P-wave is expected to occur. The stimulation device then delivers an atrial pulse prior to the next detection window, and monitors the window to determine whether a P-wave occurs therein. If a P-wave does not occur, then atrial capture is present, while occurrence of a P-wave indicates absence of atrial capture. If atrial capture is absent, the stimulation device automatically determines an appropriate atrial pacing threshold by monitoring the detection window while adjusting the stimulation pulse energy level. Advantageously, the present invention further employs a “bottom-up” adjusting scheme which starts at a low energy level, below the expected atrial pacing threshold, and increases the energy level until atrial capture is detected, thus saving energy and further avoiding corruption by large polarization signals. The latter feature is compatible with the present detection scheme and conventional evoked response detection schemes. The new atrial pacing threshold is then set at the atrial pulse level at which atrial capture was effectuated plus a predetermined safety margin.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.09/481,086, filed Jan. 11, 2000 now U.S. Pat. No. 6,408,210.

FIELD OF THE INVENTION

The present invention relates in general to implantable cardiacstimulation devices, including bradycardia and anti-tachycardiastimulation devices, defibrillators, cardioverters and combinationsthereof that are capable of measuring physiological data and parametricdata pertaining to implantable medical devices. More particularly, thisinvention relates to a system and method for automating detection ofatrial capture and determination of an atrial pacing threshold in animplantable cardiac stimulation device.

BACKGROUND OF THE INVENTION

Implantable cardiac stimulation devices (such as pacemakers,defibrillators, and cardioverters) are designed to monitor and stimulatethe heart of a patient that suffers from a cardiac arrhythmia. Usingleads connected to a patient's heart, these devices typically stimulatethe cardiac muscles by delivering electrical pulses in response todetected cardiac events which are indicative of a cardiac arrhythmia.Properly administered therapeutic electrical pulses often successfullyreestablish or maintain the heart's regular rhythm.

Implantable cardiac stimulating devices can treat a wide range ofcardiac arrhythmias by using a series of adjustable parameters to alterthe energy, the shape, the location, and the frequency of thetherapeutic pulses. The adjustable parameters are usually defined in acomputer program stored in a memory of the implantable device. Theprogram (which is responsible for the operation of the implantabledevice) can be defined or altered telemetrically by a medicalpractitioner using an external implantable device programmer.

Modern implantable devices have a great number of adjustable parametersthat must be tailored to a particular patient's therapeutic needs. Oneadjustable parameter of particular importance in stimulation devices isthe stimulus energy (i.e., the pulse amplitude and pulse width) whichcan be programmed to new values in response to changes in capturethreshold. “Capture” is defined as a cardiac depolarization andcontraction of the heart in response to a stimulation pulse. When astimulation pulse stimulates either a patient's atrium or ventricleduring an appropriate portion of a cardiac cycle, it is desirable tohave the heart properly respond to the stimulus provided. Every patienthas a “capture threshold” which is generally defined as the minimumamount of stimulation energy necessary to effect capture. Capture shouldbe achieved at the lowest possible energy setting yet provide enough ofa safety margin so that should a patient's threshold increase, theoutput of an implantable stimulation device (i.e. the pacing stimulusenergy) will still be sufficient to maintain capture. Dual chamberstimulation devices may have different atrial and ventricular pacingstimulus energies that correspond to different atrial and ventricularcapture thresholds, respectively.

The earliest stimulation devices had a predetermined and unchangeablepacing stimulus energy, which proved to be problematic because thecapture threshold is not a static value. The capture threshold also maybe affected by a variety of physiological and other factors. Forexample, certain cardiac medications may temporarily raise or lower thethreshold from its normal value. In another example, fibrous tissue thatforms around stimulation device lead tips within several months afterimplantation may cause an increase in the capture threshold. To avoidloss of capture, the earliest stimulation devices were preset to deliverpacing pulses at the maximum energy available. As a result some patientsexperienced discomfort because of the high level of stimulation.Furthermore, such stimulation pulses consumed extra battery resources,thus shortening the useful life of a stimulation device.

When programmable stimulation devices were developed, the pacingstimulus energy was implemented as an adjustable parameter that could beset or changed by a medical practitioner. Typically, such adjustmentswere effected by the medical practitioner using an external programmercapable of communication with an implanted stimulation device viatelemetry or via a magnet applied to a patient's chest. The particularsetting for the stimulation device's pacing threshold was usuallyderived from the results of extensive physiological tests performed bythe medical practitioner to determine the patient's capture threshold,from the patient's medical history, and from a listing of the patient'smedications. This improvement in adjustable pacing stimulus energypermitted programming to lower values that tended to conserve batteryenergy and extend the useful service life of the stimulation device.

Also, patients who experienced discomfort due to excessively highstimulus energy pulses could have the stimulus energy safely decreasedthus, lessening the incidence of surgical revision of the pacing system.While the adjustable pacing stimulus energy feature proved to besuperior to the previously known static stimulus energy, somesignificant problems remained unsolved. In particular, when a patient'scapture threshold changed, the patient was forced to visit the medicalpractitioner to adjust the pacing stimulus energy accordingly.

To address this pressing problem, manufacturers have developed advancedstimulation devices that are capable of determining a patient's capturethreshold and automatically adjusting the stimulation pulses to a leveljust above that which is needed to maintain capture. This approach,referred to herein as “autocapture”, improves the patient's comfort,reduces the necessity of unscheduled visits to the medical practitioner,and greatly increases the stimulation device's battery life byconserving the energy used for stimulation pulses.

A common technique used to determine whether capture has beeneffectuated is to monitor the patient's cardiac activity and to searchfor presence of an “evoked response” following a stimulation pulse. Theevoked response is an electrical event that is the response of the heartto the application of a stimulation pulse thereto. The patient's heartactivity is typically monitored by the stimulation device by keepingtrack of the stimulation pulses delivered to the heart and by examining,through the leads connected to the heart, electrical signals that aremanifest concurrent with depolarization or contraction of muscle tissue(myocardial tissue) of the heart. The contraction of atrial muscletissue is evidenced by the generation of a P-wave, while the contractionof ventricular muscle tissue is evidenced by the generation of an R-wave(sometimes referred to as the “QRS” complex when viewed on an ECGstrip).

When capture occurs, the evoked response is an intracardiac P-wave orR-wave that indicates contraction of the respective cardiac tissue inresponse to the applied stimulation pulse. For example, using such anevoked response technique, if a stimulation pulse is applied to theatrium (hereinafter referred to as an “A-pulse”), any response sensed byatrial sensing circuits of the stimulation device immediately followingapplication of the A-pulse is presumed to be an evoked response thatevidences capture of the atria.

However, it is for several reasons very difficult to detect a trueatrial evoked response. First, a high energy A-pulse may obscure theevoked response signal, making it difficult to detect and identify.Second, the signal sensed by the atrial sensing circuitry immediatelyfollowing the application of an A-pulse may be not an evoked response,but noise—either electrical noise caused, for example, byelectromagnetic interference, or myocardial noise caused by randommyocardial or other muscle contraction.

Another signal that interferes with the detection of an evoked response,and potentially the most difficult for which to compensate because it isusually present in varying degrees, is lead polarization. A lead/tissueinterface is that point where an electrode of the lead contacts thecardiac tissue. Lead polarization is commonly caused by electrochemicalreactions that occur at the lead/tissue interface due to application ofan electrical stimulation pulse, such as the A-pulse, across theinterface. Unfortunately, because the atrial evoked response is sensedthrough the same lead electrode through which the A-pulse is delivered,the resulting polarization signal formed at the electrode can corruptthe evoked response sensed by the atrial sensing circuits. Furthermore,the lead polarization signal is not easily characterized; it is acomplex function of the lead materials, lead geometry, tissue impedance,stimulation energy, and other variables, many of which are continuallychanging over time.

In each case, the result may be a false positive detection of an atrialevoked response. Such an error leads to a false atrial captureindication, which in turn leads to missed heartbeats—a highlyundesirable and potentially a life-threatening situation. Anotherproblem results from a failure by the stimulation device to detect anatrial evoked response that has actually occurred. In this case, a lossof atrial capture is indicated when atrial capture is in factpresent—also an undesirable situation that will cause the stimulationdevice to unnecessarily invoke the atrial pacing threshold determinationfunction and result in higher than necessary stimulus energy values.

Because of the problems previously stated regarding the test for atrialcapture verification and automatic threshold tests, currently availablestimulation devices do not have this capability. As a result, manymedical practitioners manually conduct atrial capture verification testsduring periodic follow up examinations. These periodic follow-upexaminations are performed by the medical practitioner after initialimplantation and configuration of the stimulation device to determinewhether the therapy delivered by the device is having the desired effectand to verify the proper operation. Capture verification and pacingthreshold assessment is typically performed by the medical practitionerusing an external programmer for controlling the stimulation devicefunctions in conjunction with a surface electrocardiogram (ECG) device.

However, this common capture verification and pacing thresholdassessment procedure is a time consuming and complex task requiringsignificant attention and effort on the part of the medicalpractitioner. The medical practitioner must spend a significant amountof time placing and subsequent removal of ECG electrodes, andconfiguring the ECG system for the patient's individual characteristics.The practitioner must also manually examine the ECG readout and analyzethe cardiac waveform to determine whether capture is present both duringinitial capture verification and during the pacing thresholddetermination tests.

It would thus be desirable to provide a system and method for enablingthe stimulation device to automatically perform atrial captureverification and atrial pacing threshold determination without a medicalpractitioner's involvement. It would also be desirable to enable thestimulation device to perform the atrial capture verification and atrialpacing threshold determination without requiring dedicated circuitryand/or special sensors. It would further be desirable to maintain arecord of atrial pacing threshold determination in the stimulationdevice so that a medical practitioner can verify the proper operation ofthe stimulation device by examining the record.

SUMMARY OF THE INVENTION

The disadvantages and limitations discussed above are overcome by thepresent invention. In accordance with the invention, a system and methodare provided for automating (1) verification of proper atrial captureaffected by atrial pacing pulses generated by a patient's implantablecardiac stimulation device, and (2) dynamic adjustment of the device'satrial pacing stimulus energy if and as necessary. The system and methodof the present invention do not require use of special dedicatedcircuitry or special sensors to implement the automated procedures. Allof the aforesaid advantages and features are achieved without incurringany substantial relative disadvantage.

The present invention is directed towards the pacing pulse generatingportion of an implantable cardiac stimulation device (i.e., abradycardia pacemaker or the pacing portion of a combinationICD/pacemaker device).

A preferred embodiment of the stimulation device includes a controlsystem for controlling the operation thereof, a set of leads forreceiving atrial and ventricular signals and for delivering atrial andventricular stimulation pulses, a set of sense amplifiers for sensingand amplifying the atrial and ventricular signals, and pulse generatorsfor generating the atrial and ventricular stimulation pulses. Inaddition, the stimulation device includes memory for storing operationalparameters for the control system, and for storing data acquired by thecontrol system for later retrieval by the medical practitioner using anexternal programmer. The stimulation device also includes a telemetrycircuit for communicating with an external programmer.

Preferably, the stimulation device of the present invention is a dualchamber rate-responsive device with atrial tracking modes (such as, DDDand DDD(R)) capable of switching modes to at least a non-tracking mode(such as, DDI and DDI(R)). Accordingly, an activity sensor is alsoincluded for sensing when the patient is at, or near, rest.

In a preferred embodiment, the control system periodically performs anatrial capture verification test and an atrial pacing capture thresholdassessment test. The frequency with which these tests are to beperformed is preferably a programmable parameter set by the medicalpractitioner using an external programmer when the patient is examinedduring an office visit or remotely via a telecommunication link. Theappropriate testing frequency parameter will vary from patient topatient and depend on a number of physiologic and other factors. Forexample, if a patient is on a cardiac medication regimen, the patient'satrial capture threshold may fluctuate thus requiring relativelyfrequent testing and adjustment of the atrial pacing threshold.

In order for the capture verification and threshold assessment tests towork properly, the patient preferably should be at, or near, rest suchthat a stable atrial rhythm can be monitored by the stimulation device.Thus, prior to initiating atrial capture verification, the controlsystem detects whether the patient is at, or near, rest using thepatient activity sensor. If the patient is not at or near rest, thecontrol system waits for a predetermined period of time beforeattempting to initiate the test again.

When the control system finally determines that the patient is at ornear rest, the atrial capture verification test is initiated by firstassessing the intrinsic atrial rate or P—P interval. The intrinsicatrial rate must be greater than the base rate such that the intrinsic,or native, P-waves are detectable. When the stimulation device ispacing, the Base Rate must be temporarily programmed to a lower value toallow the intrinsic atrial rate to emerge from the pacing rate. Thereprogramming of the Base Rate may be performed in decrements of 5 to 10ppm until a minimum lower rate, not less than 30 ppm is obtained. Thetemporary lower rate can be limited by the medical practitioner throughthe use of the programmer. If the rate of 30 ppm (or the minimumprescribed lower Base Rate of the stimulation device) is reached withoutthe emergence of an intrinsic rhythm, the capture assessment test isautomatically terminated.

With the emergence of an intrinsic atrial rate, greater than the BaseRate, the mode of operation is changed from the atrial tracking modes(such as, DDD and DDD(R)) to a non-tracking mode (such as, DDI andDDI(R)). This temporary mode change is necessary to avoid occurrence ofa Pacemaker Mediated Tachycardia (PMT) during the testing process. A PMTis a type of arrhythmia that sometimes occurs in VDD or DDD typestimulation devices, in which sensing of retrograde P-waves occurs inthe atrium and triggers the ventricle. Retrograde conduction occurs inresponse to ventricular pacing, causing atrial contraction (i.e. aP-wave). Sensing of this P-wave causes the ventricle to again bestimulated, completing an “endless” loop and thus subjecting the patientto PMT. Switching of the stimulation device into DDI mode eliminates thetriggered response in the ventricle, thus preventing the occurrence ofPMT.

After the mode switch, the control system monitors and measures thepatient's average P-wave interval over a short period of time, and thendefines an expected P-wave “window” of predetermined duration in whichP-waves are expected to occur. The control system next generates anA-pulse at a predetermined prematurity time interval prior to the nextexpected P-wave window and thereafter monitors the expected P-wavewindow to determine whether a P-wave occurs within the window. The lackof a P-wave within that window indicates that an evoked P-wave occurredas a response to the A-pulse immediately following the A-pulse (i.e.,outside the expected intrinsic P-wave window). Thus, if a P-wave is notdetected during the window, atrial capture is present. If atrial captureis thus verified, the control system switches the stimulation deviceback to original atrial tracking mode (i.e., DDD or DDD(R)) and ends theatrial capture verification test.

The presence of a P-wave within the window, on the other hand, indicatesthat there was no P-wave immediately following the A-pulse and thus noatrial capture. In this case, the control system needs to perform theatrial pacing threshold assessment test to set a new atrial pacingthreshold to re-establish atrial capture.

The control system sets atrial stimulation (i.e. the A-pulse) levelbelow the previous atrial pacing level (or at a level that is expectedto be below the patient's capture threshold), generates the A-pulse andmonitors the window for a P-wave. If a P-wave is again detected withinthe window, then the control system increments the A-pulse level andthen generates the A-pulse at the higher level while monitoring thewindow. This process continues until a P-wave is no longer presentduring the window interval.

The control system continues to monitor the window for a predeterminednumber of pacing cycles to ensure that no P-waves occur within thewindow, and then records the atrial pacing stimulus energy at thecurrent A-pulse output level as the threshold value and, optionally,adds an additional safety margin to the A-pulse threshold value. Thecontrol system records the atrial pacing threshold, the atrialstimulation levels, and other test-related data in the memory, and thenswitches the stimulation device back to original atrial tracking modebefore ending the test.

The incremental atrial pacing threshold test of the present inventionsignificantly differs from previously known approaches because atrialstimulus output is initially set lower than the current threshold andprogressively increased until capture occurs, while previously knownapproaches set initial atrial output at a high level and then decrementuntil capture is lost. The progressive output increase approach isadvantageous over prior approaches because less electrical energy isconsumed during the testing process and, moreover, because the windowobserved by the control system is not “swamped” by high output levelpulses.

In an alternate embodiment, the method of incrementally increasing theA-pulse level can also be used in an atrial capture system that employsan “evoked response” detection window following a stimulus, wherein onlya paced, or evoked, P-wave in the detection window indicates capture, asis well known in the art.

Optionally, if the patient suffers from sinus bradycardia that isaccompanied by retrograde conduction, the expected P-wave window is setto at least a predetermined portion of the cardiac cycle, and thecontrol system then searches for retrograde P-waves within the window.Similarly, presence of retrograde P-waves within the window indicatesloss of capture, while lack of retrograde P-waves confirms capture. Ifnecessary, the atrial pacing threshold is assessed and set in the samemanner as previously described.

The system and method of the present invention thus automatically verifyatrial capture and, when necessary, automatically determine a properatrial pacing threshold of the patient, without requiring dedicated orspecial circuitry and/or sensors.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and further features, advantages and benefits of the inventionwill become apparent in the following description taken in conjunctionwith the following drawings. It is to be understood that the foregoinggeneral description and the following detailed description are exemplaryand explanatory but are not intended to be restrictive of the invention.The accompanying drawings, which are incorporated in and constitute apart of this disclosure, illustrate one of the embodiments of theinvention and, together with the description, serve to explain theprinciples of the invention in general terms. Like numerals refer tolike parts throughout the disclosure.

FIG. 1 is a block diagram of a dual chamber stimulation device inaccordance with the principles of the present invention; and

FIGS. 2–4 are a logic flow diagram representing an automatic atrialcapture verification and atrial pacing threshold determination controlprogram executed by the control system of the stimulation device of FIG.1, in accordance with the principles of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The system and method of the present invention utilize a stimulationdevice's normal sensing, pulse generating and control circuitry toperform an automatic atrial capture verification and, when necessary, anatrial pacing threshold determination test.

A stimulation device 10 in accordance with the invention is shown inFIG. 1. The stimulation device 10 is coupled to a heart 24 by way ofleads 32 and 34, the lead 32 having an electrode 18 which is in contactwith one of the atria of the heart 24, and the lead 34 having anelectrode 20 which is in contact with one of the ventricles. The lead 32carries stimulating pulses to the electrode 18 from an atrial pulsegenerator 16, while the lead 34 carries stimulating pulses to theelectrode 20 from a ventricular pulse generator 22. In addition,electrical signals from the atria are carried from the electrode 18,through the lead 32 to the input terminal of an atrial sense amplifier26. Electrical signals from the ventricles are carried from theelectrode 20, through the lead 34 to the input terminal of a ventricularsense amplifier 28.

Controlling the dual chamber stimulation device 10 is a control system30. The control system 30 is preferably a microprocessor-based systemsuch as that disclosed in commonly assigned U.S. Pat. No. 4,940,052 ofMann, which is incorporated herein by reference in its entirety. Thecontrol system 30 may also be a state logic-based system such as thatdisclosed in commonly assigned U.S. Pat. No. 4,944,298 of Sholder, whichis incorporated herein by reference in its entirety. The control system30 also includes a real-time clock (not shown) for providing timingfunctionality for monitoring cardiac events and for timing theapplication of therapeutic pulses by the pulse generators 16 and 24.

The stimulation device 10 also includes a memory 14 which is coupled tothe control system 30. The memory 14 allows certain control parametersused by the control system 30 in controlling the operation of thestimulation device 10 to be programmably stored and modified, asrequired, in order to customize the operation of the stimulation device10 to suit the needs of a particular patient. In particular, the pacingstimulus energy parameters for the pacing pulses are stored in thememory 14. In addition, data sensed during the operation of thestimulation device 10, as for example during atrial capture verificationand atrial pacing threshold assessment tests, may be stored in thememory 14 for later retrieval and analysis by a medical practitionerusing an external programmer.

The control system 30 receives the output signals from the atrialamplifier 26. Similarly, the control system 30 also receives the outputsignals from the ventricular amplifier 28. These various output signalsare generated each time that an atrial event (e.g., a P-wave) or aventricular event (e.g., an R-wave) is sensed within the heart 24.

The control system 30 also generates an atrial trigger signal which issent to the atrial pulse generator 16, and a ventricular trigger signalwhich is sent to the ventricular pulse generator 22. These triggersignals are generated each time that a stimulation pulse is scheduled tobe generated by one of the pulse generators 16 or 22. The atrialstimulation pulse is referred to simply as the “A-pulse,” and theventricular stimulation pulse is referred to as the “V-pulse.” Thecharacteristics of these stimulation pulses are determined by the pacingstimulus energy settings that are stored in the memory 14.

During the time that either an A-pulse or a V-pulse is being deliveredto the heart 24, the corresponding atrial sense amplifier 26 or theventricular amplifier 28 is typically disabled by way of a blankingsignal presented to the appropriate amplifier 26 or 28 from the controlsystem 30. This blanking action prevents the amplifiers 26 and 28 frombecoming saturated with the relatively large stimulation pulses that arepresent at their input terminals during pacing pulse delivery. It alsoprevents residual electrical signals (known as “after-potentials” orpolarization) present at the electrode tissue interface from beinginterpreted as atrial or ventricular events. During the atrial captureverification and atrial pacing threshold assessment tests of theinvention, the atrial sense amplifier 26 is preferably enabled so thatP-waves may be detected during all portions of the pacing cycle.

The stimulation device 10 also includes an activity sensor 36 connectedto the control system 30 for determining whether the patient is at ornear rest. The activity sensor 36 is typically used in rate-responsivestimulation devices to alter the pacing rate to match the patient'sphysical activity. The control system 30 will only initiate the testswhen it determines that the patient is at or near rest.

A telemetry circuit 12 is further included in the stimulation device 10connected to the control system 30. The telemetry circuit 12 may beselectively coupled to an external programmer 100 by means of anappropriate communication link 112, such as an electromagnetic telemetrylink or a remote communication link such as a pair of modemsinterconnected via a telecommunications link and equipped with telemetrycapabilities.

The operation of the stimulation device 10 is generally controlled by acontrol program stored in the memory 14 and executed by the controlsystem 30. This control program typically consists of multipleintegrated program modules, with each module bearing responsibility forcontrolling one or more functions of the stimulation device 10. Forexample, one program module may control the delivery of stimulatingpulses to the heart 24, while another may control the verification ofatrial capture and atrial pacing threshold determination. In effect,each program module is a control program dedicated to a specificfunction or set of functions of the stimulation device 10. The controlprogram module dedicated to controlling the atrial capture verificationand atrial pacing threshold determination tests is described below inconnection with FIG. 2.

FIGS. 2–4 are a flow diagram representing the control program forassessing atrial capture and performing an atrial capture thresholdassessment test.

In a preferred embodiment of the invention, the control system 30periodically invokes the control program to perform the atrial captureverification test and the atrial pacing threshold assessment tests. Thefrequency with which these tests are to be performed is preferably aprogrammable parameter set by using the external programmer 100.Alternatively the programmer may be used to initiate a test sequencewhen the patient is examined during an office visit or remotely via thecommunication link 112. The appropriate testing frequency parameter willvary from patient to patient and depend on a number of physiologic andother factors. For example, if a patient is on a cardiac medicationregimen, the patient's atrial capture threshold may fluctuate, thusrequiring relatively frequent threshold testing and adjustment of theatrial pacing stimulus energy.

There are three different patient conditions during which the captureverification test and the atrial pacing threshold assessment tests maybe performed. Most commonly each patient will exhibit one or two of theconditions and rarely only all three conditions. The three conditionsusually do not exist simultaneous but may be present in combination atvarious times in the same patient. The conditions may dependent upon thepatient's daily level of activity, drug regime and time of day.Additionally, the condition may change within each patient as a functionof the progression of the disease process expressed as the indicationsfor having a stimulation device implanted and the associated symptoms.

The three patient conditions may be described as (a) an intrinsic atrialrhythm with a rate greater than the programmed Base Rate, (b) anintrinsic atrial rhythm that is over shadowed by pacing at a rategreater than the intrinsic atrial rate, and (c) a paced atrial rhythmwhere the intrinsic rate is very slow (i.e., in some patients with avery slow intrinsic rhythm, the patient may be symptomatic when paced ina DDD mode at the low atrial rate due to insufficient cardiac outputresulting in low peripheral perfusion). Accordingly, each condition willbe described separately below.

Intrinsic Atrial Rate>the Programmed Base Rate

As shown in FIGS. 2–3, and with reference to FIG. 1, when the testsequence is initiated (at step 200) and the patient's condition is onewhere the intrinsic atrial rate is greater than the programmed BaseRate, the control system 30 first determines whether the patient is at,or near, rest (at step 202). Being at rest, provides the bestopportunity for detecting when the atrial rhythm and rate are stable.During step 202, the sensor 36 provides comparative information to thecontrol system 30 to detect the rest state. If the patient is not at, ornear, rest, the initiation of the test sequence is delayed by an amountof time (at step 204) and then the test for the patient to be at, ornear rest, is reassessed.

Once rest is detected (at step 202), the mode is temporarily changedfrom an atrial tracking mode (i.e., DDD) to a non-atrial tracking mode(i.e., DDI) (at step 206). This mode change prevents the stimulationdevice from tracking a retrograde P-wave, thus preventing a PacemakerMediated Tachycardia (PMT).

The control system 30 next tests for the presence of an intrinsic atrialrate (at step 208) and measures the average P—P interval (at step 210)over sufficient period of time to verify that the patient's atrialrhythm is stable. A stable atrial rhythm and rate will consistentlyproduce P-waves in a defined detection window as determined in step 212.The detection window frequency and duration is calculated by the controlsystem 30 and is dependent upon the measured P—P interval.

The capture verification assessment test proceeds with an A-pulsegenerated (at step 214) at a predetermined “prematurity” interval, i.e.,the generated premature A-pulse will be delivered within a cardiac cycleprior to the occurrence of the P-wave detection window and after theprevious detected paced or sensed ventricular beat. The amplitude of theA-pulse is typically predetermined (i.e., programmable or set by themanufacturer). The control system 30 will monitor for P-waves (at step216) within the predetermined detection window found (at step 212).

Capture by the premature A-pulse is initially detected by the absence ofa P-wave in the detection window as determined (at step 218). If captureis detected (that is, no P-waves are occurring in the detection window,(at step 218), then the control system 30 will continue to monitor theabsence of P-waves in the detection window for, a predetermined number,“N”, of cycles and further may apply additional criteria (e.g., “F” outof “N” cycles) (at step 222).

If either a P-wave is found in the detection window (yes in step 218) orthere has not been a predetermined number of cycles without P-waves (noin step 222), then capture is not confirmed and the A-pulse will beincremented in step 220.

Once the capture has been found (at step 222), the control system 30will check to see if it is time to perform a capture threshold test, (atstep 224), to re-establish the lowest threshold.

If it is time for such a test, the A-pulse stimulus amplitude istemporarily decreased to a value expected to be below threshold (at step226) (e.g., a minimum predetermined value or a value less than thepreviously recorded threshold value). At this point the loop sequencerepeats itself: a premature A-pulse is generated (step 214); the controlsystem 30 monitors for a P-wave within the P-wave detection window (step216); if a P-wave is detected (step 218), then the A-pulse stimulusamplitude is incremented (step 220); and this loop is repeated until theabsence of a P-wave is detected (e.g., in “F” out of “N” P-waves) (step222).

If it is not time for a threshold test (no, at step 224), the controlsystem 30 continues to “C” in FIG. 3. At step 228, the control system 30determines whether a capture assessment test was performed, or simply acapture recovery for a single loss of capture. If a capture assessmenttest was performed, then the A-pulse stimulus amplitude is recorded asthe stimulus threshold value (step 232). If it was a capture recovery,then the new value of the pulse energy is recorded and used until it istime for the next capture assessment test. In either case, a safetymargin is added (step 238) and store the new A-pulse stimulus value(including the safety margin) into memory 14 (step 240). Finally, thecontrol system 30 will restore all previously programmed parametervalues (excluding, of course, the A-pulse stimulus amplitude) in step280 and end the sequence in 290 (i.e., continue with other pacingroutines).

Intrinsic Atrial Rate<the Base Rate

As shown in FIGS. 2 and 4, when the patient's condition is one where theintrinsic atrial rate is less than the programmed Base Rate, the controlsystem 30 will determine (at step 208, FIG. 2) that the stimulationdevice is pacing the atrium because the intrinsic atrial rate is lessthan the Base Rate of the stimulation device (no, at step 208).

As such, the control system 30 will proceed to step 250 (see “A” in FIG.4), and determines whether one of the following modes has beenpre-programmed based on prior knowledge of what the patient can besttolerate: (a) temporarily decrementing the Base Rate, or (b) perform aretrograde conduction test (at step 250). For the moment, thedescription below will describe option (a) and discuss option (b)thereafter.

Accordingly, the control system 30 proceeds to step 252 and temporarilydecreases the Base Rate based on the prior knowledge that thisparticular patient can tolerate a temporary lower heart rate thatoriginates from a slow atrial rate.

For the condition that the temporary Base Rate value is greater than theminimum allowable lower rate value, the control system tests for thepresence of an intrinsic P-waves (at step 254), and preferably that theP-waves repeat with consistency (e.g., by verifying that there are atleast “F” out of “N” P-waves).

When P-waves do not exist with the desired consistency (no, at step254), indicating that a paced atrial rhythm is detected, the controlsystem 30 proceeds to step 256 to determine if the new temporary BaseRate is equal to the minimum allowable value which is predetermined andstored in memory 14. If it is not, then an additional temporarydecrement of the Base Rate occurs (at step 252). This sequence isrepeated until such time as the intrinsic atrial rhythm emerges or theminimum allowable Base rate is reached.

If, the minimum temporary Base Rate is reached (at step 256), and anintrinsic atrial rhythm has not emerged, as tested at step 254, theentire test will be terminated, the original pacing mode and otherparameters are restored, and the test failure may be date and timestamped and recorded (at step 274) and the test sequence ends (at step290). The test failure information can be retrieved later via telemetrywith the external programmer 100.

When P-waves do exist with the desired consistency (yes, at step 254),the control system 30 proceeds to step 210 (“B” in FIGS. 2 and 4) andthe method steps 210–290, of establishing a detection window and forassessing whether P-waves fall within this window, then continues, asdescribed above.

Intrinsic Atrial Rate<Base Rate with Symptoms from Bradycardia

As also shown in FIGS. 2 and 4, when the patient's condition is onewhere the intrinsic atrial rate is less than the programmed Base Rate,the control system 30 will again determine (at step 208, FIG. 2) thatthe stimulation device is pacing the atrium because the intrinsic atrialrate is less than the Base Rate of the stimulation device (no, at step208).

However, based on the prior knowledge that this particular patient doesnot tolerate a temporary lower heart rate that originates from a slowatrial rate, the control system 30 will determine that a RetrogradeConduction test is need (at step 250), as previously programmed into thedevice by the physician. The control system 30 will then proceed with aRetrograde Conduction test (at step 260, in FIG. 4).

The Retrograde Conduction test is preferred when the patient mightexperience symptoms such as those that result from low cardiac outputresulting from the low intrinsic rate and it is performed at theprogrammed Base Rate. The Retrograde Conduction test begins with theatrial stimulus energy temporarily set to a desired minimum output (atstep 262). This is done to effectively simulate VVI pacing whilemaintaining the stimulation device in the DDI dual chamber modepreviously selected (at step 206).

By virtue of the determination of step 208 (FIG. 2), the stimulationdevice is currently pacing at a rate greater than the patient'sintrinsic rate and therefore the patient should be paced in theventricle without a synchronizing atrial event, paced or sensed,preceding the ventricular stimulus pulse. The lack of a precedingphysiologic encourages retrograde conduction of a signal from the pacedor naturally depolarized ventricle to the atrium. A retrograde conductedelectrical signal results in an atrial contraction, or depolarization,as evidenced by a P-wave. The presence of the P-wave establishesretrograde conduction and could only exist in response to an isolatedventricular contraction not preceded by an atrial depolarization, and aretrograde conduction pathway. An atrial stimulus of sufficientamplitude so as to cause evoke a P-wave prior to the ventricular pacedevent will block the retrograde conduction pathway such that aretrograde P-wave will not occur soon after the ventriculardepolarization.

As shown in FIG. 4, the retrograde P-wave is detected (at step 264) forseveral (e.g., “N”) beats and preferably “F” out of “N” times to ensureconsistency. If step 264 is met, the measured interval between theV-Pulse to P-wave for the retrograde conduction is determined (at step266). A retrograde detection window for the expected P-wave isestablished based on the average of a series of measured V-Pulse toP-wave intervals (at step 268).

The presence of retrograde conduction is confirmed by P-waves alwaysappearing in the retrograde detection window. Conversely, when aretrograde P-wave is not present in the retrograde detection window inresponse to an applied atrial stimulus, the absence indicates that theA-Pulse captured the atrium just prior the V-Pulse, thereby causing theretrograde pathway to be refractory to conduction. Thus, the value ofthe atrial stimulus energy when the retrograde P-wave disappears is theatrial capture threshold.

Accordingly, the atrial stimulus energy is incremented (at step 270) andwhen the presence of the retrograde P-wave is detected and, preferably,counted as “F” out of “N” cycles (at step 272), atrial capture is notfound. The atrial stimulus energy is then incremented again (at step270) and the retrograde P-wave detection and counting process continuesto loop between steps 270 and 272, until retrograde P-waves are notpresent (no, at step 272). Thus, the value of the atrial stimulus energywhen the retrograde P-wave disappears is the atrial capture threshold.

Alternatively, is the Retrograde Conduction test does not satisfy thedesired (e.g., “F” out of “N”) criteria in step 264, the test isterminated (at step 274), the original pacing mode and other parametersare restored, and the test failure may be date and time stamped andrecorded in memory 14 (at step 274) and the test sequence ends (at step290). The test failure information can be retrieved later via telemetrywith programmer 100.

The invention is not limited by the embodiments described above, whichare presented as examples only, but can be modified in various wayswithin the scope of protection defined by the appended patent claims.

1. In an implantable stimulation device, a method for performing anatrial capture verification test, the method comprising: monitoringpatient activity; detecting a rest condition based on the patientactivity; upon detection of a rest condition; sensing intrinsic P-wavesand measuring an intrinsic P—P interval; and defining an intrinsicP-wave detection window based on the measured P—P interval, andtriggering the atrial stimulation pulse prior to the intrinsic p-wavedetection window such that non-capture is defined when a P-wave issensed during the intrinsic P-wave detection window and capture isdefined when a P-wave is not sensed during the intrinsic P-wavedetection window; stimulating the heart at a predetermined time prior tothe intrinsic P-wave detection window; and determining that atrialcapture exists based on an absence of a sensed P-wave in the intrinsicP-wave detection window; wherein defining further comprises adjustingthe stimulation rate so that intrinsic P-waves occur, determining anaverage intrinsic P—P interval, and defining the detection window basedon a range about the average intrinsic P—P interval.
 2. An implantableheart stimulation device comprising: an activity sensor that sensespatient activity and generates corresponding activity signals; a secondsensor that senses cardiac activity and generates corresponding signals;a pulse generator that generates pacing pulses to be delivered to theheart; and a controller that is coupled to the activity sensor, secondsensor, and the pulse generator, wherein the controller is operative todetect a rest condition based on the activity signals, and wherein thecontroller is responsive to detection of a rest condition to perform anatrial capture verification test using the pulse generator and secondsensor; wherein the controller is responsive to receipt of signals fromthe second sensor to define the intrinsic P-wave detection window,wherein the controller is operative to control the pulse generator tostimulate the heart at a time prior to the intrinsic P-wave detectionwindow, and wherein the controller is operative to determine that atrialcapture exists based on an absence of a sensed P-wave in the P-wavedetection window; and wherein defining further comprises adjusting astimulation rate so that intrinsic P-waves occur, determining an averageintrinsic P—P interval, and defining an intrinsic P-wave detectionwindow based on a range about the average intrinsic P—P interval.
 3. Thedevice of claim 2, wherein the controller is operative to perform acapture threshold test.
 4. An implantable heart stimulation devicecomprising: means for monitoring an activity level of a patient; meansfor detecting a rest condition of the patient based on the activitylevel; and means for performing an atrial capture-related test inresponse to detection of the rest condition; wherein the atrialcapture-related test comprises adjusting a stimulation rate so thatintrinsic P-waves occur, determining an average intrinsic P—P interval,and defining an intrinsic P-wave detection window based on a range aboutthe average intrinsic P—P interval; and a controller adapted to controla pulse generator to stimulate the patient's heart at a time prior tothe intrinsic P-wave detection window, and wherein the controller isoperative to determine that atrial capture exists based on an absence ofa sensed P-wave in the P-wave detection window.
 5. The device of claim4, wherein the means for performing comprises means for performing acapture verification test.
 6. The device of claim 4, wherein the meansfor performing comprises means for performing a capture threshold test.